Pneumatic rubber tires can be prepared with a rubber tread composed of diene-based, sulfur curable, elastomer(s) which can be quantitatively reinforced with silica and a minimal amount of carbon black, if any. The tire tread is designed to be ground-contacting and is usually of a lug and groove or a rib and groove design. Such designs are well known to those skilled in such art.
The tire may also be of a cap/base construction in which the tread cap is the outer portion of the tread designed to be ground-contacting with the associated lugs and grooves and/or ribs and grooves and the tread base underlies the tread cap and is positioned between the tread cap and the supporting tire carcass. Such tire construction is well known to those skilled in such art.
Rubber by itself, without added ingredients, is generally considered as being substantially an electrical insulator or, in other words, a rather poor conductor of electricity.
A carbon black reinforced rubber vehicular tire, while still providing a degree of resistance to flow of electricity, has a considerably higher electrical conductivity, or lower resistance to flow of electricity, than rubber without the carbon black reinforcement.
Silica is, basically, a relatively poor conductor of electricity and thus a substantially silica reinforced rubber tire tread which contains only a minimal amount, if any, of carbon black reinforcement is believed to be a relatively good electrical insulator, particularly as compared to a quantitatively carbon black reinforced rubber tire tread.
Therefore, it is desirable to provide a suitable path of relatively low electrical resistance between the tire bead portion and the tread outer surface for such a tire having a quantitative silica reinforced rubber tread and minimal, if any, carbon black reinforcement.
While the dissipation of generated electrical energy may not be completely understood, it is believed that, in one aspect and insofar as a vehicular tire is concerned, electricity may be transmitted primarily from the metal rim, of steel or aluminum, for example, thence on or through the carbon black reinforced rubber surface of the tire carcass to the outer surface of a carbon black reinforced rubber tread and thence to the ground.
As used herein, the terms "quantitatively reinforced with silica", "quantitative silica reinforced rubber" and the like are generally used in conjunction with a tire tread, and with a rubber tire tread cap, in a tread cap/base construction, which contains about 30 to about 100, sometimes preferably about 30 to about 90 phr, of silica, and which may also optionally contain carbon black in which the carbon black is present in not more than about 30 phr. Often it is preferred that the ratio of silica to carbon black is at least 2/1 and sometimes at least 10/1.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber". In the description herein, rubber and elastomer are used interchangeably.
In the description herein, the term "vulcanized" or "vulcanizable", may, on occasion, be used interchangeably with the terms "cured" and "curable".